Sound reproducing system



June 27, 1933. J DREYER, JR 1,915,926

SOUND REPRODUCING SYSTEM Filed Oct. 17, 1930 Patented June 27, 1933 UNITED STATES PATENT OFFICE JOHN F. DREYER, JB., PHILADELPHIA, PENNSYLVANIA, ASSIGNOR T0 ATWATER m1 MANUFACTURING COMPANY, OF PHILADELPHIA, PENNSYLVANIA, A COR- SOUND REPBODUGING SYSTEM Application filed October 17, 1930. Serial No. 489,360.

M invention relates to electrical sound-repro ucing systems and particularly to the control or determination of the timbre of reproduction especially for attaining faithful reproduction of speech. a

In accordance with my invention, the audio-frequency characteristic of the system is modified to insure reproduction of the higher frequencies or overtones which distingulsh 10 individual voices, and preferably the higher frequencies are gradually attenuated to depress background noises, and in addltlon, the lower frequencies are attenuated substantlal- 1y to approach the ideal or to malntain the proper ratio of the amplitudes of the low to the high frequencies for natural reproductlon of s eech.

ore particularly, an impedance in the system traversed by sound-representing ourrents is shunted by a suitably high resistance in series with a small condenser progressively and gradually to attenuate the h gher audio-frequencies, and there is included 1n series in a path traversed by the sound-representin currents, an impedance, specifically a con enser, offering substantial impedance to the lower audio frequencies. 7 More specifically, the series combination of resistance and capacity aforesaid shunts a" winding, as the secondary, of an audio-frequency transformer, and the low frequency attenuating impedance is included in circuit with an amplifier tube, specifically in the input system of a tube preceding the aforesaid transformer.

Also in accordance with my invention, there is provided a switch selectively adjustable to several positions to vary the frequency characteristic of the system to adapt it to the type or characteristics of. the speech or music to be reproduced, and in one of the positions, the aforesaid series combination of resistance and capacity is included in circuit to extend the range of high audio-frequency response and simultaneously, the low frequency attenuating condenser is included in the system to maintain the proper relation of amplitude of the low to the high" frequencies. 'My invention further resides in the system and combinations hereinafter described and claimed.

For an understanding of my invention and for illustration of a system embodying it, reference is to be had to the accompanying drawmgdn which:

ig. 1 presents diagrammatically a sound am lifying and reproducing system.

ig. 2 discloses curves referred to in discussion of the characteristics of Fig. 1.

Fig. 3 diagrammatically represents a sound amplifying and reproducing system utilizing my invention.

Fig. 4 shows curves referred to in explanation of the performance and characteristics of the system of Fig. 3.

Fi 5 is a fragmentary diagram illustratin t e general use of my invention.

eferring to Fig. 1, the sound reproducing device LS, for example, a loud speaker, is supplied from an audio-amplifying system of one or more stages and utilizing thermionic tubes. Specifically, the tube V which may be the detector tube of radio receiving apparatus or associated with an electrical pick-up device, as a microphone, a phonograph pick-up, etc., is suitably coupled to a tube V1 by the coupling resistance R between whose high potential terminal and the grid of the tube V1 is included a blocking condenser C of low impedance throughout the range of audio frequencies. The grid leak resistor R1 which may have a resist ance of the order of 60,000 ohms is connected between the grid and cathode of the tube V1 to prevent blocking of the grid by an excess negative charge. The anode circuit of the tube V1 is coupled to the input circuits of the tubes V2, V2 by a coupling transformer T, the opposite terminals of whose secondaries S are connected to the grids g of the tubes. An intermediate point of the secondary is connected, preferably through a C battery, to the cathode of the tubes V2 in the usual manner. In the anode circuit of the tubes V2, V2 is included the primary P1 of a transformer T1, whose secondary S1 is connected to theloud speaker LS, for

example, to its movable coil L, when thespeaker is of the dynamic type.

To control the characteristic of the system or more specifically to control the timbre of reproduction of sound by the loud of the condensers may progressively decrease in the order named, for example, the condensers may have capacities of .01, .004 and .001 microfarads, respectively.

The effect of the capacities upon the characteristic of the system may best be understood by reference to Fig. 2 in which the curves L, M, N, and 0 respectively generally represent the audio frequency characteristic of the system as the condensers C1 to G4 are successively included in circuit. It is po1nted out that in all cases the higher audio frequencies are sharply discriminated against -or not reproduced at all with the result that speech is not naturally reproduced as the higher overtones which distinguish the individual voices are absent or greatly suppressed. a

' It is the purpose of my invention to modify the characteristic of the system particularly to adapt it for the reproduction of speech and preferably to retain the flexibility of the system to permit the characteristic to be modified by the listener for the reproduction of music according to the individual taste.

For one of the positions of the switch X, for example, when it is in engagement with the contact I), (Fig. 3) there is connected in shunt to the secondary S the series combination of resistance R2, which is of suitably high value, for example, of the order of 60,000 ohms, and the condenser C3 of suitably small value, for example, .002 microfarads. The effect of this series combination is to extend the high frequency of the audio characteristic of the system, as illustrated by the right-hand end of the curve P, Fig. 4, so that the higher frequencies are reproduced.

To maintain the proper ratio of the higher and lower frequencies, the lower audio frequencies are preferably also simultaneously attenuated, as shown by the left-hand portion of the curve P. This may be conveniently effected by inserting in series with the condenser G, as shown in Fig. 3, a condenser c of substantially less capacity. Suitable values of capacity of condensers C and 0 are .1 microfarads and .006 microfarads respectively. The condenser C, or C plus 0 in series, feeds audio-frequency currents to the resistor R. The series combination results in a gradual attenuation of the lower audiofrequencies. The additional capacity 0 may normally be shunted by the switch E operable b the switch X or some structure movable t erewith, so that, when the switch X is in engagement with contact D, the switch E is open to include the capacity a eflectively in circuit. "Accordingly, when the switch X is moved into engagement with'contact D by an operator, the system is adjusted to adapt it particularly for the reception of speech, as the higher overtones of the voice are reproduced, and the lower frequencies attenuated substantially to effect the proper ratiobetween the higher and lower voice frequencies for naturalness of reproduction. As in the system shown in Fig. 1, the contact S may be adjusted selectively to include any.

of the condensers C1, C2, or C3 in circuit for adapting the system for the reception of a other than speech, for example, to obtain the desired mellowness or brilliance of reproduc-.

tion.

In Fig. 4: graphic comparison is made of mum reproduction of high frequencies and it is apparent that'whereas the curve 0 at the high frequency end is steep and cuts ofl? the higher audio-frequencies, the curve P is of more gradual slope and insures reproduction of all frequencies within the audio range.

While I have illustrated a preferred arrangement for effecting the desired result, it will be understood that my invention comprehends specifically different arrangements within the scope of the appended claims. Referring to Fig. 5, the inductanceL is generally representative of any impedance in the system, which is traversed by the soundrepresenting currents, for example, a speaker winding; the primary or secondary of an output transformer, or of a coupling transformer between amplifier tubes, or between a pickup device and a tube; and auto transformer or coupling inductance, etc. The series combination of resistance R2 and condenser 03 shunts the impedance L, and may be either on the input or output circuit if it is a two coil transformer, to effect, as above described, an extension of the high end of the range of the audio-frequency response with gradual attenuation of the higher andhigher frequencies. Efi'ectively in series with the im-- utilized, and then preferably mechanically 'inter-related if it is desired to include or,

exclude condenser c, and the combination B2, C3 into or from the system at will.

an impedance traversed by sound-representing variations of current, a non-resonant, series combination of high fixed resistance and small capacity in shunt to said impedance for extending the range of high audio-frequency response and gradually to attenuate the higher audio-frequencies, a transfer path in said system traversed by the sound-representing variations of current, and an impedance in said path of substantial magnitude at the lower audio frequencies for at tenuating the lower audio-frequencies.

3. A system comprising a thermionic tube, means for impressing sound-representing variations of current upon the input circuit thereof, a transformer in the output circuit of said tube, a second tube, an input circuit therefor including the secondary of said transformer, an output system for said second tube including a sound-reproducing device, and means for determining the timbre of sound reproduced by said device comprising an impedance in the input circuit of the first tube of substantial magnitude for the lower audio frequencies and a series combination of capacity and resistance in shunt to the secondary of said transformer.

4. A system comprising a thermionic tube, means for impressing sound-representing variations of current upon the input circuit thereof, a transformer in the output circuit of said tube, a second tube, an input circuit therefor including the secondary of said transformer, an output system for said second tube including a sound-reproducin device, a condenser of substantial impe ance for the lower audio-frequencies, a series combination of high resistance and small capacity, and switching means for including said condenser in the input circuit for the first tube and said series combination in'the input circuit of the second tube in shunt to the secondary of said transformer.

5. A system comprising a thermionic tube, means for impressing sound-representing variations of current upon the input system thereof, a condenser of substantial impedance for the lower audio-frequencies in said input system, a switch normally shunting said condenser. a second tube, a coupling impedance between the out ut circuit of the first tube and the input circuit of the second tube, a series combination of high resistance and low capacity, and a member operable simultaneously to open said switch and to connectsaid series combination in shunt to said impedance.

6. A system comprising a thermionic tube, means for impressing sound-representing variations of current upon the input system thereof, a condenser of substantial impedance for the lower audio frequencies in said input system, a switch normally shunting said condenser, a second tube, a coupling lmpedance between the output circuit of the first tube and the input circuit of the second tube, one or more capacities, a series combination of resistance and capacity, and a member opera ble to different positions selectively to include said capacities or said series combination of resistance and capacity in shunt to said impedance and at least in one of said positions effecting opening of said switch.

7. A system comprising a thermionic tube, means for impressing sound-representing variations of current upon the input system thereof, a condenser of substantial impedance for the lower audio frequencies in said input system, a switch normally shunting said condenser, a second tube, a coupling impedance between the output circuit of the/first tube and the input circuit of the second tube, one or more capacities, a series combination of resistance and capacity, and a member operable to different positions selectivel to include said capacities or said series combination of resistance and capacity in shunt to said impedance and to open said switch when in position to effect connection of said series combination.

8. A system comprising an audio-frequency amplifier, a sound-reproducing device in the output system thereof, means for extending and depressing the high frequency end of the amplification-frequency characteristic of said amplifier, means for depressing the low frequency end of said characteristic, and means opera 1 said means in said amplifier system concurrently to extend and depress the high end of the frequency characteristic and to depress the low end of the characteristic substantially to maintain the proper ratio of the amplitudes of the low and high frequencies.

9. An audio amplifier system comprising a thermionic tube having input and output systems, a couplingtransformer includedinone of said systems, and means for extending the range of response to high audio-frequencies with gradual attenuation of the higher audio frequencies comprising a. series combination of small fixed capacity and high fixed resistance in shunt to a winding of said transformer.

JOHN F. DREYER, JR.

e simultaneously .to include both of- 

